Constant on-time (COT) control is widely used in power supply area because of its good transient response, simple structure and smooth mode transition. However, for switching converters using COT control, there may be sub-harmonic oscillation at the output voltage if the estimated serial resistance (ESR) of the output capacitor is not big enough.
To eliminate the sub-harmonic oscillation, a ripple injection method is usually employed. FIG. 1 is a block diagram of a prior constant on-time switching converter. As shown in FIG. 1, a resistor Rr1 and a capacitor Cr1 are used to generate a ramp signal which is in phase with the current flowing through the inductor L. Through a capacitor Cr2, the ramp signal is coupled to a feedback signal FB indicative of the output voltage VOUT to eliminate the sub-harmonic oscillation.
For the prior constant on-time switching converter shown in FIG. 1, the value of Rr1, Cr1 and Cr2 are determined by the input voltage VIN, the output voltage VOUT and the switching frequency of the switching converter. That means, once the application is changed, the value of Rr1, Cr1 and Cr2 should also be changed. This apparently makes circuit design complex. Furthermore, the ramp signal shown in FIG. 1 is sensitive to noise, which harms line and load regulation of the switching converter.